Connective Tissue Functions and Components

Module 4.3 Connective Tissues

• Connective tissues are divided into two basic groups and differ in both cell types and ECM components. Categories are connective tissue proper and specialized connective tissue.

Connective Tissue Functions

• Anchor tissue layers in organs and link organs together, also known as connecting and binding.
• Bones and cartilage support the body.
• Bone tissue protects internal organs; cartilage and fat provide shock absorption; components of the immune system are throughout connective tissues and provide protection.
• Blood is fluid connective tissue, and the primary transport medium in the body, for transport.

Connective Tissue Components

• Consists of cells and extracellular matrix (ECM).
• Cells are surrounded by protein fibers and embedded in the ground substance.
• ECM is a crucial feature that plays an extensive role in connective tissue type function.

Connective Tissue Proper

• Widely distributed throughout the body and connects tissues and organs.
• A component of the internal architecture of some organs and is known as general connective tissue.

Connective Tissue Proper Cells

• Resident cells inhabit tissue permanently, and migrant cells migrate depending on the current situation.
• Fibroblasts are a type of cell.
• Adipocytes are a type of cell.
• Mast cells are a type of cell.
• Phagocytes are a type of cell.
• Other immune system cells are a type of cell.

Connective Tissue Proper Cells continued

• Fibroblasts are the most common resident cell, and they have mature cells with properties like immature "blast" cells. Fibroblasts make protein fibers and ground substance (components of ECM). Fibroblasts continually produce collagen proteins.
• Adipocytes (fat cells) are present in many different connective tissues, and the cytoplasm is filled with a single large lipid inclusion.
• Mast cells are the largest resident cell and immune system cells filled with cytosolic inclusions (granules) of inflammatory mediators (histamine). Mast cells release mediators (degranulate) when stimulated, causing inflammation that activates the immune system.
• Phagocytes are immune system cells, and they ingest foreign substances, microorganisms, and dead or damaged cells by phagocytosis. Macrophages (resident or migrant) and neutrophils (migrant cells) are also phagocytes.
• Other immune system cells are available and migrate depending on the body’s current needs.

Types of Connective Tissue Proper

• Loose connective tissue, dense connective tissue, reticular tissue, and adipose tissue.

Connective Tissue Types Explained

• Loose connective tissue (areolar tissue) includes mostly ground substance; contains all three types of protein fibers, fibroblasts, and occasionally adipocytes, suspended in ground substance. Located beneath epithelium of skin, in membranes lining body cavities, and within walls of hollow organs and contains/supports blood vessels vital to avascular epithelial tissues, and houses immune system cells.
• Dense connective tissue (fibrous connective tissue) is made of mostly protein fibers and comes in three classes.

Categories of Dense Connective Tissue

• Dense irregular connective tissue is primarily disorganized collagen bundles. It is strong and resists tension in all three planes of movement, especially in high tension areas such as the dermis (deep to the skin) and surrounding organs and joints.
• Dense regular connective tissue is organized into parallel collagen bundles that are subjected to tension in one plane of movement within tendons and ligaments.
• Dense regular elastic connective tissue (elastic tissue) and has mostly parallel-oriented elastic fibers, plus randomly oriented collagen fibers in the walls of organs that stretch to function, such as large blood vessels and certain ligaments.
• The arrangement of fibers in both the dense regular and irregular connective tissues is an example of structure-function core principle.
• Reticular tissue is mostly made of reticular fibers produced by fibroblasts (reticular cells). Reticular tissue forms fine networks that support small structures such as blood and lymphatic vessels. Reticular tissue is in lymph nodes and the spleen, forming web-like nets that trap old and foreign cells, and forms part of the basement membrane. Reticular tissue supports all epithelia and the internal structure of the liver and bone marrow.
• Adipose Tissue (Fat Tissue) consists of fat-storing adipocytes and is surrounded by fibroblasts and ECM. Adipocytes can increase to a point where fibroblasts and ECM are not visible, and its functions include fat storage (major energy reserve), insulation, and shock absorption/protection.

Adipose Tissue and Obesity

• Obesity is an excess of adipose tissue in proportion to lean body mass, and there are two forms.

Forms of Obesity

• Hypertrophic is when lipid inclusions accumulate fatty acids, increasing in size up to 4x normal size. The number of adipocytes remains unchanged.
• Hypercellular is generally more severe, with the number of adipocytes increasing. Hypercellular correlates with developing obesity in infancy or early childhood (not adulthood), and adult adipocytes cannot divide to form new cells.
• Both of these forms increase the risk of health problems. The development of related disorders is complex and depends on the distribution of adipose tissue and genetic factors.

Specialized Connective Tissues

• Specialized connective tissues have specified functions.
• Cartilage, bone tissue (osseous tissue), and blood are specialized connective tissue.
• Cartilage is located in joints between bones, ear, nose, and respiratory tract segments.
• Bone tissue supports the body, protects organs, provides attachments for muscles to allow movement, stores calcium, and contains bone marrow to store fat and produces blood cells.

Additional Information on the Types of Specialized Connective Tissues

• Cartilage is a tough, flexible tissue that supports the body. Cartilage absorbs shock and resists tension/compression/shearing forces. Its ECM consists of collagen and elastic fibers, proteoglycans, and glycosaminoglycans.
• Two cell types populate cartilage: chondroblasts and chondrocytes. Chondroblasts are immature cells that divide by mitosis and produce most of the ECM. Eventually, chondroblasts surround themselves in small cavities (lacunae) in the ECM to become mature, largely inactive chondrocytes.

Cartilage Properties

• Essentially avascular, unlike most connective tissues; blood supply is limited to the outer sheath (perichondrium) of dense irregular collagenous connective tissue.
• Oxygen and nutrients must diffuse from blood vessels in the perichondrium through the ECM to chondroblasts/chondrocytes. It limits the thickness of living cartilage.
• Cartilage can be of three types: hyaline cartilage, fibrocartilage, and elastic cartilage; categorized by ECM composition.

Cartilage Types Defined

• Hyaline cartilage, the more abundant cartilage, has ECM made of ground substance with small bundles of fine collagen. It gives the tissue a glossy bluish-gray appearance present on ends of bones in joints (articular cartilage), linking sternum to ribs, framing sections of respiratory tract, and in the nose.
• Fibrocartilage is filled with bundles of collagen fibers, so there is little room for substance in the ECM and is located between bones of fibrous joints, forming articular discs that improve the fit of bones in joints.
• Elastic cartilage has mostly elastic fibers in the ECM. The tissue vibrates. Elastic cartilage is limited to one part of the body, with the external ear assisting with the detection of sound in the air and the larynx assisting with the production of sound.

Bone

• The ECM contains about 35% organic components (collagen fibers and ground substance (osteoid)), and the remaining 65% is inorganic calcium phosphate, making bone one of the hardest substances in the body.
• Bone is dynamic tissue with the capability of being remodeled. Remodeling occurs in regions of stress or inactivity. Tension increases osteoblast activity/bone deposition, while compression increases the osteoclast activity and bone resorption.

Bone Cells

• Osteoblasts (bone-builders) are on the outer surface of the bones associated with a dense irregular collagenous connective tissue covering (periosteum).Osteoblasts perform bone deposition by synthesizing/secreting organic ECM plus chemicals to trap calcium in the ECM.
• Osteocytes are osteoblasts that have surrounded themselves with ECM in lacunae. They are known as mature cells, mostly inactive, and continue making/secreting substances important for bone maintenance.
• Osteoclasts are large, multinucleated bone destroyers that perform bone resorption by secreting hydrogen ions and enzymes that break down both inorganic and organic ECM.
• Blood’s ECM is fluid; unlike most, the ECM is fluid (plasma).
• Not like fibers found in other connective tissues, the smaller plasma proteins have functions including the transport of materials and blood clotting.
• Erythrocytes (red blood cells) bind to and transport oxygen throughout the body.
• Leukocytes (white blood cells) function for immunity in the body.
• Platelets are cell fragments and play a role in blood clotting.

Explore the primary functions, components, and cell types of connective tissues. This includes understanding the roles of fibroblasts, mast cells, and adipocytes, as well as the tissue's contribution to immune function and nutrient transport. Ideal for anatomy and physiology students.